An interferometric measuring device in which such a fiber probe is used is presented in German Patent No. DE 102 44 553. That document describes an interferometric measuring device for detecting the shape, the roughness, or the distance from the surface of a measured object using a modulation interferometer, to which a short-coherent radiation is supplied by a radiation source and which has a first beam splitter for splitting the supplied radiation into a first partial beam conducted via a first arm and a second partial beam conducted via a second arm, the light phase or light frequency of one of the partial beams being shifted with respect to the other by a modulation device and this beam passing through a delay segment, the two partial beams being subsequently combined in a further beam splitter of the modulation interferometer, having a measuring probe spatially separated from the modulation interferometer and connected or connectable thereto via a fiber optic device in which the combined partial beams are split into a measuring beam conducted to the surface by a fiber optic probe device having an oblique object-side exit surface and a reference beam and in which the measuring beam (r1(t)) reflected on the surface and the reference beam (r2(t)) reflected on a reference plane are superimposed, and having a receiving device and an analyzing unit for converting the radiation supplied thereto into electrical signals and for analyzing the signals on the basis of a phase difference. The angle of inclination (y) of the exit surface is at least 46° with respect to the normal to the optical probe axis.
According to FIGS. 5 and 6 shown in the document, the oblique exit surface of an object-side optical fiber probe unit projects over a tube-shaped receptacle, so that the outstanding optical fiber probe unit, i.e., the fiber end piece, can be introduced into narrow cavities for interferometric measuring functions, for example.
The depth of the cavity to be tested is limited by the length of the fiber end piece. In optical fiber probe units, the length of the fiber end piece for a typical fiber diameter of 20 μm is approximately 2 mm.
As FIGS. 3 through 6 of the above-named document show, a semitransparent area, known as a reference layer, is introduced into the fiber and splits the incident light beam into a measuring beam and a reference beam, the reference beam being reflected back into the fiber. Such a reference layer is currently manufactured in such a way that the fiber is divided into two parts and each part is glued onto a so-called ferrule. The end face of a ferrule with the glued-in fiber is provided with a partially reflecting coating; subsequently both ferrules are connected using a guide sleeve in such a way that the unreflected component of the incident light is able to pass into the second part of the fiber. The two ferrules together with the guide sleeve form a mechanical receptacle for the fiber. The reference layer is located within the mechanical receptacle.
The maximum distance between the reference layer and the focus of the measuring beam is predefined by the path difference registered in the upstream interferometer. For typical interferometer systems this maximum distance is approximately 25 mm. The length of the fiber end piece which may be introduced into a bore hole for measuring purposes is limited to this value less the distance between the reference layer and the end of the mechanical receptacle. Extending the fiber end piece is only possible after appropriately modifying the upstream interferometer, which, however, is complicated and expensive.
Deeper bore holes may only be tested if they have such a large diameter that the mechanical receptacle may also be introduced, or alternative systems having a small tube with built-in optical components, for example, may be used, the above-named systems also having a diameter greater than 1.3 mm.
It is therefore an object of the present invention to provide an optical fiber probe which enables interferometric testing of cavities having small diameters and great depths with high measuring accuracy. The present invention furthermore relates to a method for manufacturing such an optical fiber probe.